PROJECT SUMMARY/ABSTRACT Children with autism spectrum disorder (ASD) are at increased risk for internalizing symptoms?which are the leading burden of disease worldwide among children?and their families are at greater risk of experiencing family conflict and marital discord. However, researchers have not systematically investigated the link between lower quality family relationships and children's internalizing symptoms. Moreover, there is also a knowledge gap in how heterogeneity in ASD symptom severity may place some children with ASD at increased risk for internalizing symptoms. These knowledge gaps are critical because, until they are filled, society's ability to understand the complexities of, and ultimately prevent, comorbid internalizing symptoms will remain beyond reach. The long-term goal is to reduce the rates of comorbid internalizing symptoms in children with ASD. The objective in this application is to explicate the longitudinal link between family relationships?which are often affected in families raising a child with ASD?and children's comorbid internalizing symptoms. The central hypothesis is that accounting for heterogeneity in ASD symptom severity and investigating children's responses to negative family interactions will increase understanding of how and for whom lower quality family relationships pose a risk for internalizing symptoms. The rationale for the proposed research is that its successful completion will contribute new knowledge critical for developing interventions that are targeted toward children most at risk for elevated comorbid internalizing symptoms and address mechanisms most salient for children with ASD. The following two specific aims will be pursued: 1) Identify those children with ASD who are most vulnerable to exhibiting comorbid internalizing symptoms in the context of lower quality family relationships based on ASD symptom severity; and 2) Identify the extent to which mechanisms underlying the longitudinal association between lower quality family relationships and comorbid internalizing symptoms, previously identified in typically-developing children, extend to children with ASD, and the extent to which ASD symptom severity moderates these relations. The PIs' preliminary data indicates strong feasibility of recruiting a heterogeneous sample of children with variability in ASD symptom severity, and assessing children's responses to family interactions. Building on this data, 130 children with ASD and their parents will participate in a two-wave longitudinal study over 6 months. Teachers will complete an online survey about the child at each wave. The approach is innovative because it examines heterogeneity in ASD symptom severity as a key moderating variable for identifying those children with ASD most at risk for elevated comorbid internalizing symptoms, and in comparison to the status quo of research in this area, the study utilizes a longitudinal, multi-method, multi-informant design. The proposed research is significant because this knowledge is expected to have translational importance in the prevention of internalizing symptoms in children with ASD.